Lymphokines can be released in vivo into the circulation of mice that are sensitized with tubercle bacilli and later challenged with old tuberculin; such mice were resistant to challenge with virulent tubercle bacilli. Different strains of mice vary in their capacity to produce lymphokines in vivo. The in vivo release of seven serum-lymphokines (namely, migration-inhibitory factory, type II interferon, macrophage-activating factor, lymphotoxin, mitogenic-factor, chemotactic factor, and resistance factor) will be measured in different strains of mice and this release of lymphokines in a particular strain of mouse correlated with that strain's capacity for resistance to each of the following four opportunistic fungi, namely, Candida albicans, Cryptococcus neoformans, Aspergillus fumigatus, and Rhizapus oryzae. Lymphokine-containing sera from strains of mice that produce one or two lymphokines in vivo will be used for purification and isolation of a particulr lymphokine by adsorption procedures and by subsequent physicochemical separation. Such purified lymphokines will then be used for immunotherapy by passive administration in strains of mice deficient in that particular lymphokine and challenged with one of the opportunistic fungi.